Clad metal and process for producing the same



June 4, 1935. L, F, EHRKE 2,003,481

CLAD METAL AND PRocEss Fon PRoDUcING THE SAME Filed Feb. e; 1932 Aim'INVENTOR L. EHF/(E vprocess for rmly andv permanently Patented `lune 4,v1935 UNITED STATES PATENT 4OFFICE CLAD METAL AND PROCESS FOR PRODUCINGTHE SALIE Pennsylvania Application February 6 2 Claims.

This invention relates to clad metals and to a uniting unlike metals. Inits more specic aspect the invention is directed to an X-ray anode andto a process for producing 'an X-ray 'anode of high electrical andthermal conductivity, 'having a target rmly embedded therein with theinterfaces at the planes of contact of said target, and said anodeproper being free from blow holes and pipes and having high thermal andelectrical conductivity characteristics.

X-ray tubesin general comprise an enclosing envelope containing aplurality of oppositely disposed electrodes therein. One of theelectrodes,

commonly termed the cathode, is ordinarily in the form of a lamentarybody adapted to be heated to thermionic temperatures by means of anexternal source of electrical supply, as for example the secondaryWinding of a low voltage transformer. The other electrode of said tube,commonly termed the anode, generally comprises a solid body of copper orother similar high heat and electrical conducting material, having atarget, such as tungsten or the like, embedded in the face thereofopposite said cathode.

In order that X-rays of the desired hardness may be produced, there isapplied between the incandescent cathode and the cold" anode a voltageof suicient magnitude to cause the electrons to migrate with sufficientvelocity from the cathode to said anode. These electrons strike thetarget to produce X-rays; the hardness of the X-rays so produceddepending upon the voltage applied between these electrodes.

In the commercial operation of tubes of this.

character it is frequently necessary that the volt age between theelectrode terminals be above 20,000 volts and sometimes as high as500,000 volts.

It is Well known that when the electrons emanating from the cathode aredirected to and strike the target, the target becomes heated. When theheat so developed in said target, due to the electron bombardmentthereof, is not readily dissipated in some manner, the target willbecome overheated and ultimately fused, which is obviously undesirable.

This particular phenomenon has heretofore been appreciated by those inthe art, as evidenced by the anode constructions heretofore proposed forthe purposes of attempting to provide an anode containing a target, andwhich has high electrical and heat conducting characteristics.Heretofore a number of processes have been proposed for producing a cladmetal and especially 1932, Serial No. 591,353 (Cl. fil-70.2)

a unitary structure comprising a refractory metal embedded incomparatively high thermal' and electrical conductivity metal, such ascopper or the like.

One of the methods heretofore proposed for this purpose contemplateduniting a low melting point metal of high electrical and heatconductivities, such as copper, to a relatively high melting point, highresistance metal, such as tungsten. The specific method suggested foruniting these two metals so as to obtain a juncture of good thermalconductivity and `ffirm mechanical char.- acteristics involved thefollowing steps:

. A refractory metal, as for instance tungsten in the form of a button,is placed in a suitable crucible and surrounded with a large mass ofcopper metal. This crucible containing the refractory metal and coppermetal is placed in a reducing furnace containing an atmosphere ofhydrogen gas therein. In the presence of this hydrogen, reducing,ambient the temperature of the crucible together with its constituentsis elevated to a temperature higher than that necessary to fuse thecopper so that it ows over the tungsten and in such a fluid conditionthat it may wet the same. Thereafter the resultant mass is allowed tocool and the bi-metallic structure is extracted from the crucible.

It has been found that the bi-metallic structure, generally in the formof a slug, produced in this manner, exhibits at least one desirableproperty, namely that of a strong mechanical union at the joint. Thisbi-metallic slug, however, although exhibiting this desirable property,is not suitable for all purposes and especially for `X-ray anodepurposes, because of the internal ,physical characteristicsof the copperof said slug. Upon shearing the copper of said slug either lengthwise orcrosswse it is generally found that its internal structure has arelatively large number of small voids, sometimes simulating a honeycomband commonly termed blow holes and/or one or more large cavities,commonly referred to as piping.

One other method heretofore proposed for the same general purposecontemplates the substitution of the hydrogen reducing ambient with anordinary atmospheric ambient. Bi-metallic slugs comprising a refractorymetal, as for example tungsten, having its bottom and sides coated witha low melting point metal, as for example copper, vproduced according tothis method, have not been satisfactory for X-ray anode purposes,although the copper of said slug is a homogeneous mass substantiallyfree from blow holes or piping.

Although the employment of the latter method overcomes the disadvantagesinherent in the method utilizing the hydrogen ambient so as to eliminatethe voids or cavities, it is found that the resulting product lacks theadvantages of firm mechanical union between the rare refractory metaland the copper which is provided by employing the method using ahydrogen ambient.

It is apparent that in an oxidizing medium .the tungsten will becomecoatedl with an external thin skin or film of oxide which prevents acomplete wetting of the said tungsten at the surfaces of contact betweensaid tungsten and said copper. This is my explanation of the lack of arm, tenacious, mechanical union between said tungsten and said copper.The oxide film also acts as a heat and electrical insulator to preventthe heat and electrical energy from being very quickly conducted betweenthe tungsten and the copper, the latter of which has good heat capacityand electrical conductivity characteristics.

Still another method proposed for the same general purpose comprisesplating -a tungsten button with a metal, such as nickel, iron or Monelmetal, then uniting the plated button to a copper backing by supplyingsufficient heat and pressure to thoroughly unify the intermediatebinding metal with the tungsten and the copper. The purpose of thisintermediate binding metal is to aid in the prevention of the looseningof the tungsten button from the copper backing due to the differences ofthe co-efcients of expansion between the tungsten and copper.

It is obvious that electroplating a metal, such as tungsten, with nickelthat at least some portions of the tungsten will become oxidized uponbeing immersed in the nickel bath and that when nickel is depositedthereon that someof the hydrogen that may be deposited at the cathodewill become entrapped between said nickel electro-deposited particles inthe course of electro-deposition of said nickel.

This process has other disadvantages among which may be noted thefollowing: The heat conductivity of the nickel is not as great as copperand, therefore, an anode constructed in this manner embodies anextraneous element, nickel, whose conductivity both electrically andthermally is materially below that of copper.

A still further method suggested for the same general purposes comprisesrolling platinum on a sheet of copper and brazing the sheet to aconducting mass. This process obviously necessitates the utilization ofa soft metal as an X-ray target. Besides being so limited the soconstructed anode depends for its union upon a brazing material whichincludes an alloy of copper and zinc.

When an anode so constructed is employed in an X-ray apparatus the lowmelting point metal, as for example the zinc, becomes vaporizedtherefrom and is present within the tube either as a vaporized materialor condensation product upon the interior walls of the envelope todisturb the electrical characteristics of the tube. Moreover, because ofthis general breakdown of said uniting material the target becomesdetached from the anode proper.

In order that these diculties as Well as others may be obviated, I havedeveloped the following invention which contemplates the combination ofthe rst two described processes in a novel and unique manner to providea clad metal, and

especially one adapted to be employed as the anode of an X-ray tube.

Briefly stated one aspect of my invention comprises coating a refractorymetal, such as tungsten or the like, with a relatively thin lm or layerof a low melting point metal, such as copper, in a reducing atmosphereof hydrogen or the like, thereafter adding to this bi-metallic structurea relatively large quantity of a low melting point metal, such ascopper, the latter step being carried out in air.

Another aspect of the invention is to provide a clad metal comprising arare refractory such as tungsten or the like, secured to a low meltingpoint metal, such as copper, the juncture of said refractory and lowmelting point metal being tenacious and providing a firm mechanicalunion having high thermal and electrical conductivity characteristics,with the low melting point metallic mass being substantially free fromblow holes and pipes.

An object of my invention is to provide an element comprising arefractory metal and a relatively low melting point metal, the lowmelting point metal being substantially free from blow holes or pipes,the joint between the low melting point metal and the refractory metalbeing tenacious and of high thermal and electrical characteristics.

Another object of my invention is to provide an element comprisingtungsten and copper, the copper being substantially free from blow holesor pipes, the union between said tungsten and copper being mechanicallystrong and of high thermal and electrical conductivity characteristics.

Another object of my invention is to provide an X-ray anode comprisingan element composed of a rare refractory metal and a low melting pointmetal, the low melting point metal being substantially free from voidsor pipes, the juncture between said refractory and low melting pointmetals being tenacious and of high thermal and electrical conductivitycharacteristics.

Another object of my invention is to provide an X-ray anode comprising atungsten plate embedded in a copper backing, said copper backing beingsubstantially free from pipes or blow holes and the union at theinterfaces of said metals being mechanically strong and of high thermaland electrical conductivity characteristics.

Another object of my invention is to provide a relatively simple processfor producing an element composed of a refractory metal and a lowmelting point metal, the contact areas of said two metals forming astrong mechanical joint and the low melting point metal beingsubstantially free from blow holes or pipes.

Another object of my invention is to provide a process for producing anelement composed of tungsten and copper, the union between the tungstenand copper being mechanically strong and of high thermal and electricalcharacteristics, the copper being substantially free from blow holes orpipes.

Other objects and advantages of my invention will become readilyapparent from the follow description taken in conjunction with theappended drawings wherein Figure 1 represents a vertical cross section,with some of th'e parts in elevation, of a furnace employed in carryingout my invention.

Figure 2 represents a cross sectional view of process; and

- ing material.

supports on the base of said furnace is a crucibleA the product obtainedfollowing at one stage'of my DIOCGSSI yFigure 3 represents'a verticalcross section of another furnace employed in carrying out my invention;

Figure 4V represents a cross sectional view o f the productv obtained ata later stage of my Figure 5 represents a vertical cross section withapart thereof in elevation of an X-ray anode embodying my invention.

Although my invention is broadly directed to clad metalsand to theprocess of producing the same, for the sake of simplicity of descriptionand ease of understanding and because it embodies X-ray anodes and isespecially applicable thereto, I will describe my invention as appliedto an X-ray anode and to the process of producing thesame.

According to my invention I take a rare refractory metal .I, such astungsten or the like, of appropriate size, and for X-ray purposes in theform of a button orplate, and cleanse, by any suitable means, all of thesurfaces thereof and especially the surfaces to be coated with a highelectrical and thermal conductivity low melting point metal, preferablycopper.

The surfaces to be coated with copper are preferably subjected to aninitial mechanical, chemical, or other treatment in order that they maybe substantially freed from any oxide or other coating which mightprevent the copper from forming a tenacious bond thereat. Thereafter inorder that I may obtain this tenacious bond between the tungsten and themetal, copper, which is to be united therewith and so that the juncturebetween the` adjacent faces of the tungsten, and the copper may be ofhigh' heat and electrical conductivity placed and coated with va thinlayer of molten copper in an atmosphere of hydrogen.

One means suitable for carrying out this step and employed herein onlyso that a cleark understanding of my invention may be ascertained,comprises a furnace I0 having gas tight walls and composed of anysuitable refractory heat insulat- Resting on a plurality of spaced Theinterior diameter of this ,crucible is slightly and preferably about 1A;to of an inch greater` than the diameter of the button, which in thiscase happensr to be of a circular peripheral configuration. Snuglyfitting along the sides of this crucible and having its lower surfacelocated on the upper surface of the base of this crucible is a removableplate I2, composed of the same mate` rial as the crucible per se.

Embedded in this plate I2, diametrically op posed and spacedfrom thecenter thereof a distance approximately half that ofthe diameter of thebutton I, are a bent wires I3, preferably of the same composition assaid button. Said crucible with the face to be exposed adjacent theupper surface of the plate I2 and is` preferably so positioned that itscenter is in approximate contiguity with the center of the plate I2.

The bent tungsten wires I3 extend towards each other and lap on to thelupper face of said button I to hold the same in'position during theperplaced upon said button'and insaidcrucible. Only characteristics'thetungsten button is plurality of relatively thin button I isplaced in the3 `a sumcient quantity thereof, however, is preferably contained thereinas is necessary upon the fusion thereof to coat said exposed surfaces ofthe button with a copper lm measuring aboutfrom to; of an inch inthickness. Prior to the I heating of said metals and at this stagehydrogen gas is continually admitted into said closed chamber through aconduit I4 and allowed to egress .therefrom through a conduitli. yHeatis now supplied to said chamber by any appropriate means, as forexample, high resistance glow bars (not shown). I tained and hydrogen isallowed to pass through the furnace by virtue of the conduit I4 and theoutlet I5, so that the copper fragments in the crucible maybeheated to atemperature sufficiently high to cause the copper tobecome sufficientlymolten to flow over button and wet said surfaces.

Instead of placing fragments of solid copper in the crucible togetherwith the tungsten button, I may first heat the button I to a temperatureof about 1150 C. in the presence of hydrogen. When this temperature isattained, suflicient molten copper I may be added directly to saidcrucible.

In the course of my experimentations with metals of t s character I havefound that copper heated to a temperature of about 1150 C. issuiiciently molten for the aforesaid purposes. This temperature may bemaintained for a short period and then the heating is removed and. the`hydrogen is turned oif at I4. The crucible, together with its contents,is allowed to cool to room temperature. The entir'e contents of thecrucible including the removable graphite plate 'IP may then beextracted therefromv en masse.

'I'he graphite plate I2 may be detached from the metallic structure byshipping the tungsten wires at the point of juncture offsaid plate andsaid button leaving a slug composed of the tungsten button I having itsbottom and side walls coated with alayer 2 of copper metal,` thejuncture of said tungsten button and said'copper metal beingmechanically strong and having a high thermal and electrical'conductivitycharacteristic.

It is found that when copper is applied to said button in this mannerthat there may sometimes'appear small pipes or blow holes extending inthe direction of the depth of said copper, and rgenerally above thesurface skin of copper in contact with the tungsten button.

Because of the thinness .of said metalthese pipes or blow holes areapparent to the eye, or the copper outer surface or skin has indicationsthereon generally locating, if not definitely establishing, the exactposition of said; blow holes or pipes. These indications on the surfaceof said copper appear as blisters or depressions. In order that the plugmay be'prepared for the-next cause the outer surface of said ,coppercoating is preferably machinedbefore the next step', Xwhich machiningopens substantially all blow holes or pipes that may be therein. Insteadof detaching the copper coated button I from the plate I2,l prior tomachining, the button I may be machined while attached to said plate.

'I'he next step of the process comprises placing this slug with itslower uncoated face adjacent the upper face of a removable plate ofgraphite or the like, located in thefbottomof a crucible 20 within afurnace 2| containing air `and free from the hydrogen reducingatmosphere,

the exposed surfaces of said 'I'he heating operation is main- 1 The slugcomprising the button I and the coppensed with by increasing the size ofthe copper per coating 2 is now heated to a temperature just casting 3,thus obviating the mechanical joint beslightly below the melting pointof copper and tween the head 5 and the rod 6. y

preferably about 1000 C. When the slug becomes Although I have describedmy invention with 5 heated to this temperature there is admitted intominute particularity, this is not to be taken by 5 said crucible abovesaid slug a quantity of molten way of limitation, but by way ofillustration and copper, at a temperature of about 1l25 C., very theinvention is to be limited only by the prior art.

large in comparison to the quantity of copper `What is claimed is:coating said slug and sufficient to form an anode 1. The plOCeSS 0fmaking Clad metals @OnSiSthead 5 of desired length which may be of anying of tungsten and copper comprising, in the 10 dimension and about oneinch. presence of hydrogen, coating said tungsten with Thereafter thecrucible together with the anode a relatively thin layer of moltencopper, solidifyhead 5 comprising the button I, the copper coating saidmolten layer, mechanically treating said ing 2 and the copper casting orbody 3 are allowed copper layer and thereafter elevating the temto coolto room temperature. Subsequently any perature of said thin layer andcoating said thin 15 appropriate machining and cleansing operationslayer with a second molten layer of copper whose may be performedthereon to provide an X-ray mass is great as compared to that of saidthin anode head comprising a rare refractory metal, layer andsolidifying said second layer. tungsten, button or target I havingadjacent 2. The process of making clad metals consistthereon a mass ofcopper which is substantially ing of tungsten and copper comprising, inthe 20 homogeneous and free from blow holes or pipes, presence ofhydrogen, coating said tungsten with the juncture between said buttonand said copa relatively thin layer of molten copper, solidper beingtenacious and forming a firm mechaniifying said thin layer, mechanicallytreating said cal, high heat conductivity, electrical conductivcopperand thereafter elevating the temperature ity union therebetween. of saidthin layer to slightly below its melting 25 This anode head 5 may thenbe machined and point and then adding thereto a molten laye:` mountedina copper rod 6 to provide a suitable of copper whose mass is great ascompared to X-ray anode shown in Figure 5. If desired the, that of saidthin layer and solidifying said copper. machiing of the anode head 5 andsubsequently mounting the same in a copper rod 6, may be dis- LOUIS F.EHRKE. 30

